1. Field of the Invention
The present invention relates generally to a process for making honeycomb sandwich panels, and more particularly pertains to an improved process which is particularly applicable to glass and forms a resultant glass honeycomb sandwich panel in a single furnace cycle.
2. Discussion of Prior Art
Honeycomb sandwich panels are a common structural form, combining high rigidity, low weight and low thermal inertia, and are a preferred structure for glass mirror substrates. Their property of low thermal inertia, and the resulting absence of convective distortion, has recently been recognized as a fundamental advantage for large ground-based optical systems. There is also a practical advantage of low thermal inertia in that high quality optics of inexpensive glasses with non-zero expansion coefficients can be readily figured, without long delays involved in allowing the mirror to achieve a state of thermal equilibrium during testing thereof. Applications such as mirror substrates take advantage of the good stability, homogeneity and low thermal expansion that can be achieved in glass.
Presently available processes for the manufacture of glass honeycomb sandwich structures are rather elaborate. Typically the honeycomb interior is prepared separately by cutting, fitting and fusing on a piece by piece basis. In greater detail, glass mirrors with a monolithic honeycomb structure have been formed in the prior art from a very low expansion titanium silicate glass by welding "L" shaped pieces to make a square cell structure. This technique of making the honeycomb is laborious, frequently taking months of effort to complete, and is not suitable for most glasses because of the impracticality of welding together glass pieces with appreciable thermal expansion coefficients. After assembly, the fragile open honeycomb must be precisely machined to fit the faceplates, and is then fused or frit bonded between the face places. High material and manufacturing costs prohibit the use of such glass honeycomb sandwich structures in all but the most expensive optical systems, such as in large diffraction limited satellite cameras.
Mirror substrates with a monolithic internal honeycomb structure have also been cast in the prior art as a single piece in a complex mold, but these structures tend to be somewhat heavy.
Lighter weight fusion bonded mirror substrates have also been constructed in the prior art, but have an internal structure of plates, tubes, or slotted struts that are bonded only to the faceplates and not to each other. This type of construction, utilized for example in the mirrors of The Multiple Mirror Telescope, does not realize the full stiffness given by a monolithic honeycomb core.
A further prior art process, perhaps the most pertinent to the present invention, is described by Angel (Proc. SPIE, 383, 52, 1983) in which a hexagonal glass honeycomb is formed from individual round glass tubes in the following manner. The tubes are first hexagonal close packed, and are then filled with free-flowing sand. Upon softening of the glass by heating, the sand acts to press the tubes outwardly against each other, causing them to fuse together into a monolithic hexagonal honeycomb. Large sheets of very regular honeycomb have been constructed in one step in this manner, but the process is slow because of the thermal inertia of the sand, and also requires subsequent machining and fusing of the faceplates to the honeycomb structure. This prior art process differs from the present invention in several significant aspects, namely, in this prior art process only a subassembly (i.e. the interior glass honeycomb structure) is formed in a single furnace cycle, and moreover, the subassembly glass honeycomb structure produced thereby must be machined prior to the application thereto of glass faceplates in a separate fusing or frit bonding operation.